Towards the use of triple ionic-electronic conducting lanthanum tungstate as a hydrogen separation membrane

This thesis investigates the potential of triple ionic-electronic conducting lanthanum tungstate (LWO, La28−xW4+xO56-δ) as a hydrogen separation membrane. LWO is a fluorite structured material with an inherently deficient oxygen sublattice. It exhibits high chemical stability against reducing and acidic paired with high protonic conductivity, making it a promising candidate as a hydrogen separation membrane, provided its electronic conductivity is enhanced. However, predicting the membrane performance remains challenging due to the limited availability of material properties, including thermodynamic parameters related to defect formation, as well as concentrations and mobilities of the involved charge carriers (H+, O2-, e-). This research aims to evaluate the phase stability, as well as the transport and thermodynamic properties of compositions derived from LWO and to predict their performance as a hydrogen separation membrane.

Chapter 1 describes the motivation and aims of the research, addresses fundamental principles of solid oxide cells, focusing on protonic conductors, and elaborates on triple ionic-electronic conductors and their potential applications in hydrogen-related technologies. In Chapter 2, the phase stability of LWO is investigated, with a particular focus on the impact of partial substitution of W by Nb, Mo, Mn, or Fe. In Chapter 3 a novel approach for evaluating both transport and thermodynamic parameters of triple ionic-electronic conducting oxides is presented. Chapter 4 presents the development and validation of a defect transport model based on the Nernst-Planck-Poisson equation set, enabling the deconvolution of the total electrical conductivity transient into its constituent partial conductivity transients. In Chapter 5, a transport model is developed for calculation of the steady-state permeation fluxes through triple ionic-electronic conducting membranes and is validated for pure and molybdenum-doped LWO membranes. In Chapter 6, the oxygen ion and proton transport properties of La5.4WO11.1-δ are investigated using electrical conductivity relaxation following hydration and dehydration steps at constant oxygen partial pressure. The results show that the magnitude of the applied current significantly affects both the electrical conductivity and the relaxation behavior. Finally, Chapter 7 provides recommendations for future research on the development of triple ionic-electronic conducting membranes for hydrogen separation.